There seems to be a bug in the touch event handler on iOS devices.
You can't manually set a label for the first word in a sentence. If the span is longer than one word, you can overcome this bug by swiping from the last span token to the beginning, but not the other way around. However, if the span is one word long and happens to be the first word in the sentence, there's no way to label it at all.
The same bug occurs both on iPhone and iPad.